Polyester resin compositions are well known in the art. Takikawa et al., for example, in U.S. Pat. No. 3,709,777, taught organic, solvent based polyester impregnating resins for decorative laminates. The resin contained: phthalic acid; mixed alcohol, including propylene glycol and an ethylene glycol type; unsaturated aliphatic dicarboxylic acid selected from one or both of maleic acid and fumaric acid; diallyl phthalate prepolymer; peroxide catalyst; release agent such as a wax; and organic solvent such as a ketone. In the area of powder coating compositions, Fujiyoshi et al., in U.S. Pat. No. 3,953,403, taught powdered polyester resins containing: terephthalic acid; glycol; dicarboxylic acid selected from maleic acid, maleic anhydride, fumaric acid, citraconic acid, mesaconic acid, itaconic acid, or the like; stabilizers and modifiers, such as benzoin, benzophenone, benzilic acid, para-phenylphenol or stearyl alcohol; cross-linking agent such as hexamethyoxymethyl melamine; and peroxide catlyst. Neither of these formulations would be particularly useful as a high bond strength polyester electrical insulating composition.
Sassano, in U.S. Pat. No. 4,391,947, provides polyester insulating compositions having good heat stability. The Sassano compositions contain: a polyester component admixture of from about 15 to 40 mole % of isophthalic acid, tetrahydrophthalic acid, and/or endo methylene tetrahydrophthalic acid; neopentyl glycol as sole aliphatic dihydric alcohol; unsaturated aliphatic dicarboxylic acid; dual inhibitor mixture of hydroquinone and mono tertiary butyl hydroquinone; solubilizer such as triethyl phosphate; and 25 to 35 mole % of copolymerizable monomer, such as vinyl toluene; all mixed with from 0 to about 10 parts of an optional melamine-formaldehyde resin, per 100 parts by weight of polyester component. This provides an outstanding solventless, low viscosity, dipping and impregnating polyester. However, even further improvement of flash point and bond strength at high temperatures, and catalyzed storage stability at temperatures over 30.degree. C. are required for some electrical insulating resins in the class F-H thermal capability range, especially for dip applications.